Yes, VMS Platinum 7 supports 5.1. From the Help file:
"With Vegas and DVD Architect, you can create your own 5.1 surround projects and burn them to DVDs: use a 5.1 surround project to edit 5.1 surround clips from a DVD camcorder or create your own 5.1 channel project by adding stereo audio and then adding audio to the additional surround tracks."
That sounds reasonable. The front left & right channels can be a single stereo track. The rear left & right channels likewise can be a single stereo track.
That being said, i don't know if Vegas Studio will import a 5.1 AC3 file and keep it as 6 channels. However, if you have Vegas Studio, it should only take a few moments to try it and see what happens.
From what I've read in the help files, VMS is expecting a camcorder to provide 4 separate tracks, front L&R and rear L&R. From memory, if you import 5.1 AC3 you'll see all 6 channels.